1. Field of the Invention
The present invention relates to a magnetic encoder and a method for manufacturing such a magnetic encoder, wherein the magnetic encoder may be used in conjunction with a sensor on a semiconductor chip that is placed opposite the magnetic encoder, and is capable of producing codes as represented by a sequence of pulses that are generated by magnetic forces. More particularly, the present invention relates to a magnetic encoder that includes a supplemental or reinforcing ring member and a ring-like magnetic rubber member, wherein the magnetic rubber member is obtained by vulcanizing a raw rubber in its unvulcanized state, and magnetizing the rubber so that the rubber member has S poles and N poles alternately around its circumference. The magnetized rubber member is firmly combined with the supplemental ring member and is uniformly magnetized in its circumferential direction, and accordingly, the magnetic encoder can provide strong magnetic forces. Furthermore, the present invention provides a method for manufacturing such a magnetic encoder.
2. Description of the Prior Art
Conventional magnetic encoders that include a rubber material possessing magnetism have been manufactured by a number of methods, some examples of which are described below.
According to one method, a proper quantity of magnetic ferrite powder is added to a raw rubber material in its unvulcanized state (which may also be referred to as xe2x80x9can unvulcanized raw rubber materialxe2x80x9d), and the powder and rubber material are mixed together. Then, a resulting mixture is formed into a rubber sheet in an unvulcanized state by performing a sheet rolling process. The unvulcanized raw rubber sheet is then cut into slit-like square strips. Each of the square strips is then joined annularly at opposite ends thereof so that a ring-like rubber blank is formed. The ring-like rubber blank thus obtained is then placed in a cavity in a metal mold, where the rubber blank is compressed, while applying heat thereto, so that the rubber blank is formed into a rubber member having a round circumference. Finally, the rubber member is magnetized so that S poles and N poles appear alternately around its circumference.
According to another method, an unvulcanized raw rubber material that contains a magnetic ferrite powder is extruded into elongated strips by using an extruding machine. Then, each of the strips is temporarily joined annularly at opposite ends thereof so that a ring-like rubber blank is formed. The ring-like rubber blank thus obtained is then placed in a cavity in a metal mold, where the rubber blank is compressed, while applying heat thereto, so that the rubber blank is formed into a rubber member having a round circumference. Finally, the rubber member is magnetized so that S poles and N poles appear alternately around its circumference. This method is widely used since it is expected to enhance moldability and workability.
According to still another method, a raw rubber material that contains a magnetic ferrite powder is rolled into elongated sheets by using a roll machine. A rubber blank having an annular shape is stamped out from each of the sheets by using a shearing machine. The rubber blank having the annular shape is then magnetized so that S poles and N poles appear alternately around its circumference. Finally, the rubber member thus obtained is attached to a supplemental or reinforcing ring by virtue of an adhesive.
The conventional methods that have been mentioned above have respective problems, which will be described below.
In the first method mentioned above, at an initial stage where the magnetic powder, such as ferrite, is added to and mixed with the raw rubber material, it is not considered that the magnetic powder should be aligned regularly in a particular orientation when the rubber material is magnetized. Accordingly, when the rubber material containing such magnetic ferrite powder is magnetized, magnetic forces are produced that are not aligned regularly in a circumferential direction. It is therefore impossible to obtain a magnetic encoder that provides powerful and uniform magnetic forces in the circumferential direction when the encoder becomes magnetized.
In the second method mentioned above, although that part of the magnetic powder, such as ferrite, that exists in a middle portion of the strip may be aligned regularly in a particular orientation, remaining parts of the magnetic powder that exist at joined ends tend to be aligned irregularly in a circumferential direction. Therefore, it is also impossible to obtain a magnetic encoder that provides powerful and uniform magnetic forces in the circumferential direction when the encoder becomes magnetized.
In the third method mentioned above, a rubber ring member that has S poles and N poles alternately around its circumference is attached to a supplemental or reinforcing ring member by virtue of adhesive, after the rubber ring member is magnetized. It is therefore difficult to firmly join the rubber ring member and the supplemental or reinforcing ring member into a single unit.
In light of the problems of the prior art methods described above, it is therefore one object of the present invention to provide a magnetic encoder that includes a rubber ring member having S poles and N poles alternately arranged in a circumferential direction, and a supplemental or reinforcing ring member, wherein the rubber ring member and the supplemental or reinforcing ring member are firmly joined into a single unit, such that the magnetic encoder provides powerful and uniform magnetic forces in the circumferential direction when the encoder is magnetized.
Another object of the present invention is to provide a method for manufacturing such a magnetic encoder.
According to the method of the present invention, and the magnetic encoder of the present invention obtained by the method, the magnetic encoder may be used with a sensor on a semiconductor sensor chip that is placed opposite the magnetic encoder, and may produce codes as represented by a sequence of pulses generated by magnetic forces. The method, as well as the magnetic encoder obtained by the method, will be described below in some detail by referring to accompanying drawings.
In the method for manufacturing the magnetic encoder in accordance with the present invention, an unvulcanized raw rubber material is first provided, to which material a magnetic ferrite powder is added, and the rubber material and ferrite powder are mixed together. A resulting mixture of the unvulcanized raw rubber material and magnetic ferrite powder is passed through a rolling or extruding machine that forms the mixture into a sheet blank 1 that contains the magnetic ferrite powder aligned regularly in a particular orientation, as shown in FIG. 1. Altematively, a sheet blank 1 can be produced by extruding the mixture using an extruding machine, then by passing an output of the extruding machine through a rolling machine so as to form the sheet blank 1. In this case, the sheet blank 1 that has been extruded and then passed through the rolling machine may also contain magnetic ferrite powder aligned regularly in a particular orientation.
Next, the sheet blank 1 in either case is stamped across a planar direction. This results in a ring-like sheet blank or annular blank 11 as shown in FIG. 2.
The ring-like sheet blank 11, along with a supplemental or reinforcing ring 4, is placed on a metal mold, specifically between lower and upper halves 2, 12 of the metal mold as shown in FIG. 3. Then, the metal mold is operated to compress the ring-like sheet blank 11 and supplemental ring 4 in an axial or vertical direction while applying heat thereto. This compression provides a vulcanizing action that forms a ring-like rubber member 6 which is a composite annular member including the ring-like sheet blank 11 and supplemental ring 4, which are joined together by the vulcanizing action.
The ring-like rubber member 6 thus obtained may then be magnetized so that S poles and N poles appear alternately around its circumference, as shown in FIG. 4. This results in a magnetic encoder 5 according to the present invention.
In the magnetic encoder 5 obtained according to the method of the present invention, the ring-like rubber member 6, on which S poles and N poles are alternately arranged in a circumferential direction, is attached to the supplemental ring 4 through the vulcanizing action. Thus, the magnetic encoder 5 obtained by the method according to the present invention includes the ring-like rubber member 6, which is vulcanized, and supplemental ring 4 that are joined together more firmly than when two similar members are joined together by using adhesive as in a prior art method.
When a sheet blank 1 is formed according to the method of the present invention, it is very important that the sheet blank 1 is formed so that it can have magnetic ferrite powder aligned regularly in a particular orientation. The reason for this is that if the magnetic ferrite powder contained in the sheet blank 1 is aligned regularly in such a particular orientation as described above, then a magnetic encoder 5 that includes a vulcanized ring-like rubber member 6, derived from such sheet blank 1, can reduce any error in magnetic pitch precision that might otherwise occur, and each of S poles and N poles that are alternately magnetized can provide regular magnetic flux density.
As used in this specification, the term xe2x80x9cerror in magnetic pitch precisionxe2x80x9d refers to any error between an actual value and a theoretical value in terms of distance between an N pole and an S pole, when the N pole and the S pole are alternately magnetized in a circumferential direction of the vulcanized ring-like rubber member 6.
By considering the above factors, it is desirable that the sheet blank 1 should be formed from any of the following manners.
When a rolling machine is used to form the sheet blank 1, it is desirable that unvulcanized raw rubber that contains magnetic ferrite powder is passed through a series of rolls so that the rubber is gradually formed into a sheet.
When an extruding machine is used to form the sheet blank 1, it is desirable that the extruding machine has an outlet port having a simple flat configuration.
When an extruding machine is used, the extruding machine may alternatively have an outlet port having a ring-like shape, and may provide a hollow cylindrical shape output. Then, an output of the extruding machine is cut along the length thereof, or in a longitudinal direction. This results in an elongated sheet blank 1.
As an alternative method, it is desirable that unvulcanized raw rubber that contains magnetic ferrite powder is first extruded by a extruding machine having a simple flat outlet configuration, or a circular outlet configuration, so that a flat-like output or a strip-like output may be formed, and this flat-like output or strip-like output is then passed through a rolling machine so that a sheet blank 1 is formed.
In any case, it is desirable that the sheet blank 1 has a thickness of between 0.1 mm and 3.00 mm, and that magnetic ferrite powder contained in the sheet blank 1 is aligned regularly in a particular orientation. In this way, a magnetic encoder 5 that includes vulcanized ring-like rubber member 6, derived from such sheet blank 1, can reduce any error in magnetic pitch precision that might otherwise occur, and has S poles and N poles each of which provides a regular magnetic flux density when these poles are alternately magnetized around a circumference of the ring-like rubber member.
It should be noted that the sheet blank 1, which contains the magnetic ferrite powder that is aligned regularly in the particular orientation, may desirably have a thickness of between 0.1 mm and 1.00 mm. In this case, error in magnetic pitch precision can be further reduced, and magnetic flux density of each of the S and N poles alternately magnetized can become more regular.
If it is requested to obtain a desired thickness, several sheet blanks 1, each of which has a thickness of between 0.1 mm and 3.00 mm, or preferably between 0.1 mm and 1.00 mm, may be stacked one on another. Thereby, the desired thickness can be obtained.
In order to measure any error in magnetic pitch precision, a magnetic encoder 5 that is obtained as described above is tentatively placed opposite a sensor on a semiconductor sensor chip 7 by setting a gap 8, between the encoder and sensor chip, to each of values that range between 0.5 mm and 3.00 mm, as shown in FIG. 5. In each case, it was found that an error that occurred represented less than 1.5% of all measurements made. It was also found that each of the S and N poles provided regular magnetic flux density.
It is understood from the foregoing description that any of the methods described above comprise the steps of: forming a raw rubber blank in an unvulcanized state, and containing magnetic ferrite powder, into a sheet blank 1 so that the sheet blank has a thickness of between 0.1 mm and 3.00 mm, preferably between 0.1 mm and 1.00 mm, and also has the magnetic ferrite powder aligned regularly in a particular orientation; stamping the sheet blank 1 across a planar direction into a ring-like blank or annular blank 11 without disturbing alignment of the magnetic ferrite powder as already established; and compressing the ring-like blank 11 in an axial or vertical direction, while applying heat thereto, so that the magnetic ferrite powder flows in the planar direction, thereby causing the magnetic ferrite powder to be aligned more regularly. These steps may interact with each other so that obtained is a magnetic encoder in which any error in magnetic pitch precision is reduced, and which has S poles and N poles each providing a regular magnetic flux density.
Thus, a magnetic encoder 5 manufactured by performing the above steps may be used in any application that requires magnetic pitch precision error to be limited to less than 1.5%, when the magnetic encoder is spaced apart from a sensor on a semiconductor sensor chip 7 with a gap 8 of 0.5 mm to 3.00 mm between the encoder and sensor chip, as shown in FIG. 5.
The magnetic encoder 5 manufactured by performing the above steps may also be used in any application that requires the sensor to provide analog output, since the magnetic encoder 5 can limit magnetic pitch precision error to less than 1.5%, and can reduce any irregular magnetic flux density for each of S and N poles when these poles become alternately magnetized in a circumferential direction.
In summary, vulcanized ring-like rubber member 6 may be obtained by vulcanizing a ring-like blank or annular blank 11 that contains magnetic ferrite powder aligned regularly in a particular orientation, such that in the vulcanized ring-like member 6 there is little irregularity in alignment of the magnetic ferrite powder. Thereby, after the vulcanized ring-like rubber member 6 is magnetized, and S poles and N poles alternately appear around a circumference of the rubber member, as shown in FIG. 4, a magnetic encoder 5 is obtained which provides powerful magnetic forces while reducing irregularity of magnetic forces in a circumferential direction.
The method according to the present invention allows magnetic ferrite powder contained in unvulcanized raw rubber to remain aligned regularly in a circumferential direction from a time when the unvulcanized raw rubber is formed into a ring-like rubber blank or annular blank 11 until a time when the rubber blank is joined to a supplemental ring 4 during a subsequent vulcanizing process. When the ring-like rubber member 6 thus obtained is then magnetized so that S poles and N poles appear alternately around a circumference of the rubber member, provided are powerful and uniform magnetic forces at any circumferential point. Thus, a magnetic encoder 5 including such a ring-like rubber member 6 can perform very well.
Because the ring-like rubber blank 11 is joined to the supplemental ring 4 during the vulcanizing process, the rubber blank and supplemental ring are joined together more firmly into a single unit.